This invention is directed to commercial or residential heat pump systems that provide heating or cooling of a comfort zone, as required, and which can also provide water heating. The invention is more particularly directed towards an improved system of controlling both the sensible temperature and latent temperature of air in a comfort space, employing the operating features of a variable speed heat pump employed for space cooling.
Integrated heat pumps are often employed for comfort heating or cooling, as needed, to a residential or commercial comfort space, i.e., the interior of a residence, office complex, hospital, or the like. Integrated heat pumps can also be employed to heat water. A heat pump system for air conditioning, comfort space heating, and water heating is described in U.S. Pat. No. 4,766,734. Systems of this type can have a number of modes of operation, such as air conditioning alone, space heating alone, water heating alone, air conditioning with water heating, and comfort space heating with water heating. Additional modes, such as a defrost cycle, can also be employed. For comfort space heating and water heating, resistive elements are employed as auxiliary heating elements for use at times when the heat pump alone cannot produce sufficient heating for the comfort space or produce enough hot water in the water heater.
During early morning hours and in the late evening, there may be unusually high humidity accompanied by a low sensible cooling load. At those times control for the heat pump system by temperature alone can often produce insufficient dehumidification.
In the cooler parts of a summer day, the indoor comfort space air can feel somewhat stuffy because of increased relative humidity. However, if the thermostat setpoint is lowered to produce equivalent comfort for persons in the comfort space, the heat pump system will produce over-cooling, and operate at less than its best efficiency. Also, in the hotter parts of the day, after the outdoor relative humidity has dropped, the heat pump system will cool to the reduced setpoint, and produce cooler conditions than the target optimal comfort.
Up until now, residential and commercial heating and air conditioning systems have been controlled, primarily or exclusively, on the basis of the sensible (dry bulb) temperature. Although some equipment has included a humidity sensor or humidistat, the control strategy itself has not produced an even comfort level or efficient operation. Typically, previously proposed control schemes produce simply a reduced setpoint under high humidity conditions, rather than an effective control of both sensible temperature and relative humidity.
Modern heat pump systems and air conditioning systems can employ to their advantage variable speed drives both for their compressors and also for fans and blowers in the indoor and outdoor heat exchangers. Variable speed operation permits the system cooling capacity to be varied over a range, and this permits the system to operate at optimal efficiency. Typically, the compressor speed is adjusted according to cooling demand, so that the system operates at full capacity only when there is a large cooling load, and can be reduced to a lesser capacity, e.g. down to fifty percent of full capacity, when there is a lower cooling load.
A method of controlling a variable speed heat pump and air conditioning system to maintain a user-selected comfort level is described in U.S. Pat. No. 4,873,649. In that scheme, a comfort index is calculated based on a number of parameters including the dry-bulb temperatures, humidity, air velocity, and other factors such as carbon dioxide level and particulate count. The comfort setting is used to adjust the compressor speed, blower speed, and evaporator superheat according to a rather complex relationship involving dynamic non-linear optimization. With such a scheme, it is not possible for a user simply to select a relative humidity setpoint, such as fifty percent, and also simply to select a temperature setpoint, as a rather complex comfort index setpoint is required as the operative parameter.